---
title: "SMTP 550 errors during email verification - Emvero"
description: "A 550 reply can concern a mailbox or server policy. Read how Emvero handles ambiguous recipient refusals and why mailbox-not-found probe evidence stays unknown."
url: https://emvero.io/glossary/ambiguous-rejection/
language: en
updated: 2026-10-07
---

All terms

Glossary

# Ambiguous rejection

An ambiguous SMTP rejection refuses a recipient without establishing the cause. A 550 reply alone cannot distinguish a missing mailbox from an access or policy restriction.

## Why a 550 reply needs context

An SMTP `550` reply says that the requested action was not taken. [RFC 5321](https://www.rfc-editor.org/rfc/rfc5321.html#section-4.2.3) lists possible causes including a missing mailbox, denied access and a policy refusal. The number alone cannot tell a verifier which cause applies.

The command matters too. At `RCPT TO`, the server has heard the recipient address. A refusal before that command concerns an earlier step, such as the envelope sender. The [policy rejection explanation](https://emvero.io/glossary/policy-rejection/) covers those restrictions.

An enhanced status code can help distinguish an addressing problem, mailbox condition or policy rule. [RFC 3463](https://www.rfc-editor.org/rfc/rfc3463.html#section-3.3) defines mailbox-status conditions separately from other categories. The extra detail helps classify the probe response, but cannot establish ownership or an actual delivery outcome. The [hard and soft bounce explanation](https://emvero.io/glossary/hard-vs-soft-bounce/) covers sending failures.

## How Emvero reports recipient refusals

`SMTP_RECIPIENT_REJECTED_AMBIGUOUS` returns `unknown` with `allow_with_email_confirmation`. The current policy also converts an observation classified as `SMTP_MAILBOX_NOT_FOUND` to this result and reason. A provider profile cannot enable a separate rejection rule for that observation. [Unknown versus undeliverable](https://emvero.io/compare/unknown-kept-apart/) explains how this differs from deterministic syntax or domain failures.

`SMTP_MAILBOX_STATUS_ERROR` identifies a mailbox-status problem, and `SMTP_MAILBOX_DISABLED` identifies a disabled mailbox. These return `risky` with the confirmation recommendation. The cause is more specific, but the observation cannot tell you when the address will receive mail again.

## Choosing an application or sending action

If access matters at signup, have your application send confirmation; Emvero sends no verification message. The [signup validation guide](https://emvero.io/guides/signup-form-validation/) explains how to apply the recommendation. When reviewing a list, keep the address and refusal reason together until your sending policy resolves the decision. The [verification result guide](https://emvero.io/blog/reading-a-verification-result/) shows the complete response.

## Related reason codes

The API uses these identifiers to explain a check. The notes identify values that the current verification policy or probe does not return.

`SMTP_RECIPIENT_REJECTED_AMBIGUOUS`

Recipient refused for an ambiguous reason

`SMTP_MAILBOX_STATUS_ERROR`

Server reply indicated a mailbox-status problem

`SMTP_MAILBOX_NOT_FOUND`

Current policy reports this observation as SMTP_RECIPIENT_REJECTED_AMBIGUOUS

## Related terms

### Hard bounce and soft bounce

Permanent and temporary delivery failures describe a sending attempt. Learn how they differ from verification evidence.

### SMTP handshake

The greeting and commands used to ask a mail server about a recipient, stopping before message content is sent.

## Compared side by side

### Confidence score vs check evidence

A confidence score is not a delivery probability. Check stages, reason codes and the recommended action explain the result.

[All terms](https://emvero.io/glossary/)

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